The mammalian suprachiasmatic nucleus (SCN) is a circadian pacemaker required for daily rhythms in behavior and physiology. In vivo and in vitro, multiple circadian oscillators within the SCN synchronize to each other to sustain near 24-h rhythms. It is presently unclear if this rhythmicity is intrinsic to a specialized population of SCN neurons or what mechanisms couple their circadian rhythms. In addition, recent molecular evidence suggests that other tissues can act as circadian oscillators, but the bases for their rhythmicity and roles in behavior are unknown. The proposed studies directly address these issues by taking advantage of long-duration recording technologies--multielectrode arrays and bioluminescent reporters of gene activity--and the unique properties of mice and hamsters with mutations in genes involved in circadian timekeeping. The first Specific Aim tests the hypotheses that individual SCN neurons are autonomous, circadian pacemakers and that the pacemaking neurons are a small subset of SCN neurons. The strategy is to characterize the rhythmic ability of fully isolated SCN neurons and then, after fixation, their neurochemical content. Specific Aim 2 tests the hypothesis that SCN neurons synchronize to each other via vasoactive intestinal polypeptide and not fast synaptic communication. Using specific antagonists, agonists, and genetic knockouts, this aim complements the first aim in identifying pacemakers and the signals required for their coordinated activity. The recent discoveries of putative circadian oscillators in many mammalian tissues have led to the hypothesis that the circadian system is hierarchically organized. Specific Aims 3 and 4 will determine the function and molecular basis for circadian rhythms in the main olfactory bulb (OB), a model circadian oscillator to be compared to the SCN. Using behavioral, anatomical and physiological assays for rhythmicity in the OB, these aims will establish an in vivo role for the clock in the OB. Furthermore, they will directly test the hypothesis that at least one of the genes involved in circadian timekeeping differ between the SCN and OB. These experiments will, for the first time, identify circadian pacemakers in two brain areas, the mechanisms that coordinate their ensemble rhythms, and the distinct roles they play in behavior.